The general structures and manufacturing processes for electronic packages are described in, for example, Donald P. Seraphim, Ronald Lasky, and Che-Yo Li, Principles of Electronic Packaging, McGraw-Hill Book Company, New York, N.Y. (1988), and Rao R. Tummala and Eugene J. Rymaszewski, Microelectronic Packaging Handbook, Van Nostrand Reinhold, New York, N.Y. (1988), both of which are hereby incorporated herein by reference.
As described by Seraphim et al., and Tummala et al., an electronic circuit contains many individual electronic circuit components, e.g., thousands or even millions of individual resistors, capacitors, inductors, diodes, and transistors. These individual circuit components are interconnected to form the circuits, and the individual circuits are further interconnected to form functional units. Power and signal distribution are done through these interconnections. The individual functional units require mechanical support and structural protection. The electrical circuits require electrical energy to function, and the removal of thermal energy to remain functional. Microelectronic packages, such as, chips, modules, circuit cards, circuit boards, and combinations thereof, are used to protect, house, cool, and interconnect circuit components and circuits.
Within a single integrated circuit, circuit component to circuit component and circuit to circuit interconnection, heat dissipation, and mechanical protection are provided by an integrated circuit chip. This chip is referred to as the "zeroth" level of packaging, while the chip enclosed within its module is referred to as the first level of packaging.
There is at least one further level of packaging. The second level of packaging is the circuit card. A circuit card performs at least four functions. First, the circuit card is employed because the total required circuit or bit count to perform a desired function exceeds the bit count of the first level package, i.e., the chip. Second, the circuit card provides for signal interconnection with other circuit elements. Third, the second level package, i.e., the circuit card, provides a site for components that are not readily integrated into the first level package, i.e., the chip or module. These components include, e.g., capacitors, precision resistors, inductors, electromechanical switches, optical couplers, and the like. Fourth, the second level package provides for thermal management, i.e., heat dissipation. One type of printed circuit board is a metal core printed circuit board.
Metal core printed circuit boards are described by Nandakumar G. Aakalu and Frank J. Bolda in "Coated-Metal Packaging", in Rao R. Tummala and Eugene J. Rymaszewski, Microelectronic Packaging Handbook, Van Nostrand Reinhold, New York, N.Y. (1988), at pages 923 to 953, specifically incorporated herein by reference.
As used herein, coated metal packages, also referred to as metal core packages, are polymer encapsulated conductive metal cores. Circuitization, that is, personalization, is carried out on the surface of the polymeric encapsulant, with vias and through holes passing through the polymeric encapsulant and the metal core.
The metal core may be a copper core, or a copper-Invar-copper core. Copper and copper-Invar-copper cores spread out the heat from the devices mounted on the card or board. The high thermal conductivity allows the devices, for example the memory devices or logic devices, to operate at lower temperatures. The metal core also provides high mechanical strength and rigidity to the package. The metal core allows the substrate to carry large and heavy components, and to function in environments where shock, vibration, heat, and survivability are a factor.
Copper-Invar-copper is a particularly desirable core material because of its thermal, electrical, and mechanical properties. Invar is an iron-nickel alloy containing approximately sixty four weight percent iron and thirty six weight percent nickel. While deviations from this composition are possible, the 64-36 alloy has the lowest coefficient of thermal expansion in the iron-nickel binary system, approximately 1.5.times.10.sup.-7 /degree Centigrade.
Lamination of the Invar between copper films of controlled thickness determines the properties of the copper-Invar-copper core. This is shown in Table 1, below, adapted from Nandakumar G. Aakalu and Frank J. Bolda in "Coated-Metal Packaging", in Rao R. Tummala and Eugene J. Rymaszewski, Microelectronic Packaging Handbook, Van Nostrand Reinhold, New York, N.Y. (1988), Table 13-2, at page 932.
TABLE 1 ______________________________________ Properties of Copper-Invar-Copper Property Cu/In/Cu Cu/In/Cu ______________________________________ % Cu/% Invar/% Cu 12.5/75/12.5 20/60/20 Coefficient of thermal 44 53 expansion (.times.10.sup.-7 /deg C.) Electrical Resistivity 7.0 4.3 (micro-ohm-cm) Young's Modulus 1.4 1.35 (10.sup.5 mPa) Enlongation (%) 2.0 2.5 Tensile Strength 380-480 310-410 (mPa) Density 8.33 8.43 (grams/cm.sup.3) Thermal Conductivity x-y plane 107 160 z plane 14 18 Thermal Diffusivity 0.249 0.432 (cm.sup.2 /second) Specific Heat 0.484 0.459 (Watts/gm deg C.) Yield Strength 240-340 170-270 ______________________________________
The encapsulating polymer may be a perfluorocarbon, a phenolic, an epoxy, or a polyimide. For example, the encapsulant may be a phenolic-fiber composite, exemplified by phenolic and paper. Alternatively, the encapsulant may be an epoxy-fiber composite, illustrated by, for example, epoxy and glass fiber, and epoxy and polyperfluorocarbon fiber. According to a still further alternative, the encapsulant may be a polyimide-fiber composite, such as polyimide and glass fiber, polytetrafluoroethylene and glass fiber, or polyimide and polyperfluorocarbon fiber.
In low circuit density applications, where the vias and through holes are on a wide pitch, i.e., about ten or less per square centimeter, and are large diameter, i.e., greater than about 0.8 millimeters in diameter, simple techniques can be used to avoid shorting the vias and through holes. Thus, where the metal core package contains such vias and through holes, the polymer may be applied after the vias and through holes are drilled. This allows the deposited polymer to provide the through hole insulation.
Alternatively, the core or compensator may be encapsulated, drilled, the Cu etched back, and the vias and through holes coated with dielectric.
This is not the case with high circuit density metal core packages. One problem of high circuit density metal core packages is poor dielectric coverage of the vias and through holes. This poor coverage results in shorting of the vias and through holes by and/or through the metal core structure. This propensity to shorting results in the practical limitation of metal core packages to single sided boards, and to via-less double sided boards with edge connections.